As shown in FIG. 7, such an angular position adjusting mechanism enabled to steplessly adjust the position is configured by forming an internal gear 231 in one of brackets and also forming an external gear 221 in the other bracket so that the internal gear 231 is meshed with the external gear 221, and that these gears are mutually rotatably combined with each other, to thereby enable the adjustment of the angular position and also enable the brackets to firmly hold the position.
As shown in FIG. 3, the tooth profiles employed by these gears are formed according to a cycloid curve 274, which is the locus of a basic point 271 on the circumference of the rolling circle 80, by setting, for example, a fixed circle 71 and a rolling circle 80, which rotates while internally touching the fixed circle 71. That is, the tooth profile of the external gear 221 is set like a circular-arc centered at the position of the basic point 271 shown in FIG. 7, while the tooth profile of the internal gear 231 is formed in such a way as to have tooth flanks 275 formed at an equidistance A from a predetermined cycloid curve 274 (see JP-A-55-068328).
Incidentally, as will be described later, in a case where the fixed circle 71 and the rolling circle 80 have a relation similar to that in the aforementioned case, and where the basic point is set at a place, which is not on the circumference of the rolling circle, the locus of the basic point is called a trochoid curve that is distinguished from the cycloid curve.